The present invention generally relates to a method for forming an oxide film of a semiconductor device, and an oxide film forming apparatus. More specifically, the present invention is directed to an oxide film forming method for a semiconductor device, capable of shortening pre-processing time for concentration measurements, and also to an oxide film forming apparatus.
In stages for manufacturing variable capacitors and the like, a silicon epitaxial layer corresponding to a thin silicon monocrystal layer is grown on a silicon substrate and a plurality of silicon epitaxial layers are stacked. In such a silicon epitaxial layer, a film thickness and a film quality constitute very important factors. Conventionally, such a characteristic check is carried out by measuring impurity concentration.
An impurity concentration measurement of epitaxial layer is performed by forming an oxide film on a surface. When mercury of a mercury probe of a measuring apparatus is made in contact with the formed oxide film, the oxide film forms a depletion layer, so that a Shottky barrier diode is produced. The characteristic value of this Shottky barrier diode is measured so as to measure the impurity concentration of the epitaxial layer.
As a result, it is required to form the oxide film as the pre-process operation for the impurity concentration measurement of the epitaxial layer. In the principle of this concentration measurement, the film thickness of this oxide film must be selected to be at least 15 angstrom, preferably approximately 20 angstrom.
Conventionally, as the method for forming the oxide film on a wafer, the wafer is boiled within a hydrogen peroxide water solution. The maximum film thickness of the oxide film formed by this conventional boiling method is limited to approximately 10 angstrom. It is practically difficult to form such a film thickness thicker than about 10 angstrom.
Therefore, in addition to the above-described manufacturing stage, the resultant oxide film is blown by nitrogen gas for approximately 1.5 hours, so that approximately 5 angstrom may be additionally formed on the above-described oxide film. However, there is a problem that usually 2 hours, approximately 3 hours in maximum are required as the time required to form such an oxide film having a total thickness of about 15 angstrom in addition to the film thickness of 10 angstrom by way of the hydrogen peroxide water solution boiling process. There are further problems that there is a lack of stability in the oxide film forming stage, and the reproducibility thereof is deteriorated. Moreover, since an overall film thickness is limited to on the order of 15 angstrom, there is another problem that it is practically difficult to form a film thickness thicker than 15 angstrom. As a consequence, this conventional oxide film forming method is not proper.
Furthermore, according to the thermal oxidation method in which a wafer is oxidized in high temperatures in the diffusion furnace, when the oxide film to be formed becomes the thin film, there is another problem that the uniformity of the thin film is deteriorated due to the air entrainment. In addition, as to the film thickness, a desirable range is not always obtained.
Under such a circumstance, the oxide film forming method and the oxide film forming apparatus by way of the gas discharge have been developed. As such conventional oxide film forming method and oxide film forming apparatus, for instance, the oxidation effects of ozone generated by the discharge are utilized (disclosed in, for example, Japanese Laid-open Patent Application No.4-39931 opened in 1992, or No.7-033405 opened in 1995).
These conventional techniques have introduced the principle structure such that the AC high voltage is applied to the electrodes provided in the gas so as to produce the gas discharge. Thus, ozone is finally generated.
In other words, when the AC high voltage applied to the electrodes provided in the gas is increased to produce the strong electric field, the generations of the electron avalanche are rapidly increased, and the electrolytic dissociation, or ionization is temporarily interrupted due to the shield effect by the space charges. Soon the electron avalanche is again generated. Thus, it is broughted into a small intermittent discharge, i.e., a corona discharge state. A large amount of ozone is generated by such a corona discharge, or the silent discharge such that the occurrence of corona is suppressed by providing the insulating material on the electrode surface. Thereafter, the silicon wafer is oxidized by utilizing the oxidation effect of this ozone to thereby form the oxide film.
On the other hand, when the supply of AC power is increased to further increase the strength of the electric field from the above-described condition that the corona discharge, or the silent discharge is generated, the discharge region is limited to the specific portion of the electrode surface, and also the emission strength is increased, so that the corona, or silent discharge is transferred to the arc discharge. As a result, the supplied power is consumed so as to heat the electrodes, so that this supplied power never contributes the generation of ozone.
Accordingly, the oxide film forming operation conditions by way of the ozone oxidation are set within the range defined from such conditions that the generation of the corona discharge, or the silent discharge is commenced up to such conditions that the corona or silent discharge is transferred to the arc discharge in the above-mentioned conventional structure by using the ozone oxidation effects.
However, it is difficult to form the silicon oxide film having the film thickness thicker than, or equal to 15 angstrom by the above-explained conventional structure by using the oxidation effects of ozone produced from the corona discharge, or the silent discharge. Moreover, there is a drawback that the time required for forming a predetermined thickness of the silicon oxide film becomes very long.
In addition, this conventional structure owns such problems that the stability of the oxide film forming process is low, and the reproducibility of forming such an oxide film is deteriorated.
The present invention has been made to solve the above-described various problems and drawbacks involved in the conventional techniques, and therefore, has an object to provide an oxide film forming method of a semiconductor device, and an oxide film forming apparatus, capable of shortening pre-process time for a concentration measurement, and furthermore capable of achieving supreme reproducibility of forming an oxide film, while stabilizing a forming step.
Before describing the inventive ideas, the Inventors, or Applicants have considered the mechanism of oxide film forming stages on a surface of a silicon wafer. The following interpretation is so far established. That is, a formation of an oxide film is not progressed in such a way that silicon atoms of a silicon substrate are moved through the previously formed silicon oxide film to the surface of the silicon oxide film. But, the formation of the oxide film is progressed in such a manner that oxygen is diffused into the previously formed silicon oxide film in accordance with the temperature gradients, and then reacts with a surface (boundary) between silicon and silicon oxide (Sixe2x80x94SiO2 surface). In this case, Si may be predicted such that at the Sixe2x80x94SiO2 surface, Si is under excessive state in view of stoichiometry.
On the other hand, resistivity of monocrystal silicon oxide (crystal) is 1022 xcexa9cm, namely a high resistivity value, whereas resistivity of the silicon oxide film formed on the wafer is only about 1016 xcexa9cm. This implies that both the coupling degree and the close-packing degree of the silicon oxide film formed on the wafer are relatively low, and spaces through which atoms can easily pass are present.
As a consequence, it may be considered in the conventional structure with employment of ozone as follows. That is, ozone sequentially passes through the spaces contained in this oxide film and may reach the boundary surface. At the boundary surface, the ozone is decomposed to thereby produce oxygen molecule and oxygen in the generation period. This oxygen in the generation period may oxidize silicon of the boundary surface, so that it becomes a silicon oxide film (SiO2). This newly formed silicon oxide film is added to increase the thickness of the silicon oxide film. As a result, the boundary surface is gradually moved inside the wafer along the wafer internal direction in conjunction with the increase of the thickness of the silicon oxide film.
On the other hand, since ozone owns a large dimension in view of micrometer, the spaces through which the ozone should pass become deep (otherwise, hierarchies of spaces are increased) in connection with increasing of the silicon oxide film thickness. Then, the ozone can readily pass through the spaces. In other words, it is conceivable that the resistance when the ozone is diffused into the silicon oxide film (film diffusion resistance) is increased in accordance with increasing of the film thickness, and therefore the diffusion of this ozone is blocked, so that the ozone does not reach the boundary surface, and thus the thickness of the formed film is limited (for example, limitation in on the order of 15 angstrom).
Otherwise, originally, the oxygen molecule generated from the ozone in the boundary surface is again diffused up to the surface of the oxide film and then is emitted from the film. However, while the above-described boundary film diffusion resistance is increased, the emission of the oxygen molecule is also decreased. As a consequence, the oxygen molecule will remain near the boundary surface and in the oxide film, so that partial pressure is increased, and the diffusion of this ozone from the boundary film surface to the boundary surface is blocked by this partial pressure. As a consequence, there is a limitation in the thickness of the formed film.
Then, the Inventors have paid their attentions to oxygen ions (monoatomic negative ions, or diatomic negative ions) instead of the ozone (oxygen in generation period being used as oxidation agent) in the prior art as to the optimum mode of the oxygen atoms or the oxygen molecule, which may react with silicon.
In other words, since the dimension of the oxygen ion is smaller than that of the ozone in view of micrometer, even when the film thickness becomes deep, the film diffusion resistance is not increased. As a consequence, the oxygen ions can be easily moved in the oxide film gradient and then can reach the Sixe2x80x94SiO2 boundary surface. Among these oxygen ions, for instance, the monoatomic oxygen ions deprive electrons from silicon at the boundary surface to cause it as silicon of positive charges, and also the monoatomic oxygen ions per se become diatomic negative ions. Then, the diatomic negative ions react with this silicon of positive charges to thereby form the silicon oxide film. As a consequence, it is conceivable that the oxide film forming method by way of the ion reaction is suitable.
Furthermore, it is conceivable that an oxide film forming apparatus capable of readily and mainly generating oxygen ions rather than generating ozone, is preferable.
To achieve the above-described object, an oxide film forming method of a semiconductor device, according to the present invention, is featured by that oxygen ions are made in contact with a surface of a semiconductor substrate to thereby form a thin oxide film on the surface of the semiconductor substrate.
According to this method, since the diffusion resistance related to the oxygen ions is low, the diffusion of the oxygen ions to the boundary surface can be easily made, so that the thickness of the formed film can be increased.
In particular, when the oxide film forming method is realized such that the above-described the oxygen ions are produced by conducting oxygen gas into an electric field generated in response to a DC high voltage having a negative polarity and then by ionizing the conducted oxygen gas, the oxygen gas is ionized without producing ozone.
In addition, when the oxide film forming method is realized such that the above-described the oxygen ions are produced by causing oxygen gas to impinge on an electrode to which a DC high voltage having a negative polarity is applied and then by ionizing the impinging oxygen gas, the oxygen gas can be effectively ionized.
Also to achieve the above-described object, an oxide film forming apparatus of a semiconductor device, according to the present invention, is featured by comprising: a high voltage source for generating a DC high voltage having a negative polarity; one pair of electrodes arranged apart from each other via a space portion, the DC high voltage being applied to one electrode of the one-paired electrodes; a flow path channel formed from an upstream located in a direction perpendicular to the electrodes via the space portion to a downstream, through which oxygen gas may flow; an oxygen source for supplying the oxygen gas from the upstream side of the flow path channel; and a substrate base arranged on the downstream side of the flow path channel, by which a semiconductor substrate can be mounted at a position where this semiconductor substrate is exposed by the oxygen gas which has passed between the one pair of electrodes.
In accordance with the oxide film forming apparatus with employment of the above-described arrangement, a strong electric field is produced between one pair of electrodes in which the DC high voltage having the negative polarity generated from the high voltage source is applied to one of these electrodes. At such a stage that the oxygen gas supplied from the oxygen source will flow from the upstream of the flow path channel to the downstream thereof, for instance, when the electrons which have been produced by the electron avalanche and then have been accelerated by the electric field impinge the oxygen gas, the oxygen ions are produced. The flow path channel intersects this electric field and is provided between the electrodes. Then, the semiconductor substrate mounted on the substrate base arranged in the downstream of the flow path channel is exposed by the oxygen ions, so that the oxide film is formed on the surface of the semiconductor substrate due to ion reaction.
Moreover, when this oxide film forming apparatus is arranged by that at least the one pair of electrodes, the flow path channel, and the substrate base are stored into a chamber, coupling between the oxygen ions and the molecule contained in air is suppressed, so that the oxygen ions can be effectively made in contact with the semiconductor substrate.
Also, when the oxide film forming apparatus is arranged by that the above-described the chamber is made of a closed structure, and pressure in the chamber is increased higher than, or equal to atmospheric pressure, both the oxygen pressure and the oxygen ion pressure within the chamber are increased. Accordingly, the ionization of the oxygen gas and the ion reaction by the oxygen ions can be advanced.
Otherwise, when the oxide film forming apparatus is arranged by that the above-described the chamber is made of a closed structure, and pressure in the chamber is decreased lower than, or equal to atmospheric pressure, the molecule contained in air within the chamber is decreased. As a result, there is a little chance of molecule entrainment, and thus the film quality of the oxide film can be improved.